[Octave-bug-tracker] [bug #55643] Function "nproc" just reports the environment variable OMP_NUM_THREADS not the "current number of available processors".

[Kai Torben Ohlhus]

Follow-up Comment #9, bug #55643 (project octave):

Okay, I got confused by nproc and I am pretty sure Octave behaves correctly.

Without Octave on my system


$ nproc
4
$ nproc --all
4
$ OMP_NUM_THREADS=30 nproc
30
$ OMP_NUM_THREADS=30 nproc --all
4


Octave 4.2.2 (distro):


$ /usr/bin/octave --eval 'nproc'
ans =  1
$ /usr/bin/octave --eval 'nproc all'
ans =  4
$ /usr/bin/octave --eval 'nproc current'
ans =  1
$ /usr/bin/octave --eval 'nproc overridable'
ans =  1
$ OMP_NUM_THREADS=30 /usr/bin/octave --eval 'nproc'
ans =  1
$ OMP_NUM_THREADS=30 /usr/bin/octave --eval 'nproc all'
ans =  4
$ OMP_NUM_THREADS=30 /usr/bin/octave --eval 'nproc current'
ans =  1
$ OMP_NUM_THREADS=30 /usr/bin/octave --eval 'nproc overridable'
ans =  30


Octave 50.0.90 (self compiled):


$ octave --eval 'nproc'
ans =  4
$ octave --eval 'nproc all'
ans =  4
$ octave --eval 'nproc current'
ans =  4
$ octave --eval 'nproc overridable'
ans =  4
$ OMP_NUM_THREADS=30 octave --eval 'nproc'
ans =  4
$ OMP_NUM_THREADS=30 octave --eval 'nproc all'
ans =  4
$ OMP_NUM_THREADS=30 octave --eval 'nproc current'
ans =  4
$ OMP_NUM_THREADS=30 octave --eval 'nproc overridable'
ans =  30


A solution might be to return "nproc all" by default or a better documentation
what is returned: that is the minimum of OMP_NUM_THREADS and the detected
number of processors.  The question is, does Octave regard OMP_NUM_THREADS for
all of its computations?

File "libgnu/nproc.c":


unsigned long int
num_processors (enum nproc_query query)
{
  unsigned long int omp_env_limit = ULONG_MAX;

  if (query == NPROC_CURRENT_OVERRIDABLE)
    {
      unsigned long int omp_env_threads;
      /* Honor the OpenMP environment variables, recognized also by all
         programs that are based on OpenMP.  */
      omp_env_threads = parse_omp_threads (getenv ("OMP_NUM_THREADS"));
      omp_env_limit = parse_omp_threads (getenv ("OMP_THREAD_LIMIT"));
      if (! omp_env_limit)
        omp_env_limit = ULONG_MAX;

      if (omp_env_threads)
        return MIN (omp_env_threads, omp_env_limit);

      query = NPROC_CURRENT;
    }
  /* Here query is one of NPROC_ALL, NPROC_CURRENT.  */
  {
    unsigned long nprocs = num_processors_ignoring_omp (query);
    return MIN (nprocs, omp_env_limit);
  }
}


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